The invention relates generally to a two-dimensional magnetic recording (TDMR) disk drive and more particularly to a TDMR disk drive with multiple stacked sensors wherein different sensors are selected to provide a readback signal.
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag). One ferromagnetic layer adjacent to the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference or pinned layer. The other ferromagnetic layer adjacent to the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the pinned-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the amount of tunneling current through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
A proposed technology that uses multiple CPP-MR sensors is two-dimensional magnetic recording (TDMR). In TDMR, multiple sensors that are located on a single structure access the same or adjacent data tracks to obtain signals that are processed jointly. This results in an increase in areal data bit density. In addition to increasing areal bit density, TDMR may provide an increased readback areal density through signal processing of two signals from the multiple data tracks that are read concurrently. A structure with multiple stacked read sensors for TDMR is described in US 2013/0286502 A1.
Each of the individual CPP-MR sensors in a TDMR read head structure is required to be located between two shields of magnetically permeable material that shield the sensors from recorded data bits that are neighboring the data bit being read. During readback, the shields ensure that each sensor reads only the information from its target bits.